Abstract

Molecular-dynamics simulations were carried out for the SPC, SPCE, TIP4P, and TIP5P models of water at 298 K. From these results we determine the following quantities: the absolute entropy using the two-particle approximation, the mean lifetime of the hydrogen bond, the mean number of hydrogen bonds per molecule, and the mean energy of the hydrogen bond. From the entropy calculations we find that nearly all contributions to the total entropy originates from the orientation effects. Moreover, we determine the contributions to the total entropy which originate from the first, second, and higher solvation shells. It is interesting that the limits between solvation shells are clearly visible. The first solvation shell contributes approximately to the total entropy; the second solvation shell contributes approximately , while contributions from the third and other solvation shells are very small, approximately in summary. This indicates that water molecules are strongly ordered up to 0.55-0.6 nm around the central water molecule, and beyond this limit the ordering diminishes. The results of calculations (entropy and hydrogen bonds) are compared with the experimental data for the choosing of the best watermodel. We find that the SPC and TIP4P models reproduce the best experimental values, and we recommend these models for computer simulations of the aqueous solution of biomolecules.

Received 25 May 2005Accepted 14 July 2005Published online 09 September 2005

Acknowledgments:

The molecular-dynamics calculations were carried out at the Academic Computer Center in Gdańsk. The author also wishes to thank Professor P. Bała (Nicholas Copernicus University, Faculty of Mathematics and Computer Science, Toruń, Poland) for providing the parallel version of GROMOS96 molecular-dynamics program used in this work.

Article outline:I. INTRODUCTIONII. DETAILS OF SIMULATION PROCEDUREIII. RESULTS OF CALCULATIONSA. The entropy calculationsB. Calculations of the energy and lifetime of the hydrogen bondIV. DISCUSSION